The enzymes would still be active, but much slower, but the starch gelatinization temperature is up around 150F, so the remaining activity of these enzymes would be even less effective. I don't know how much less, however.

That might be fine if you let it go longer, but there are other activities that would continue at a lower temperature. One problem might be some issues with proteinases and glucanase working to make the beer seem thin. The primary concern I can think of, however, is that bacteria would grow at low temperatures and with longer mashes. Perhaps you might manage to convert enough sugar to make a Berliner Weisse if you tried mashing cool (120F or less?) for a few days or more, but I wouldn't try it on anything that you didn't want to be sour.